The present invention relates to a motor-driven power steering system, and more particularly to a motordriven power steering system for use on a motor vehicle, including a power steering unit comprising an electric motor for generating an assistive torque to reduce the driver's effort to turn the steering wheel, the power steering unit being controllable in response to the speed of travel of the motor vehicle.
Power steering systems for motor vehicles such as automobiles include hydraulically operated or motor-driven power steering units. Generally, the steering torque required to turn the steering wheel is small when the motor vehicle runs at higher speeds, and is large when the motor vehicle travels at lower speeds. Therefore, it is desired that the power steering unit be operated only when the motor vehicle runs at lower speeds, requiring an assistive wheel turning torque, and the power steering unit be at rest when the motor vehicle runs at higher speeds.
Japanese Patent Publication No. 53-38849, published on Oct. 18, 1978, discloses one conventional moror-driven power steering system. The disclosed motor-driven power steering system includes a clutch for selectively connecting an assistive-torque generating motor to the shaft of a steering wheel. The clutch is controlled such that when there is no steering torque applied, i.e., when the motor vehicle runs straight or the steering wheel is turned across its neutral position, the clutch is connected at the time the speed of travel of the motor vehicle is reduced, and is disconnected at the time the speed of travel of the motor vehicle is increased. The steering torque is therefore prevented from abruptly varying while the steerable wheels of the motor vehicle are being steered.
The problems with the conventional motor-driven power steeing system are as follows: When the speed of travel of the motor vehicle is reduced and there is some steering torque applied, as when the motor vehicle is decelerated while turning along a curve, the clutch remains disconnected and hence a large manual force is required to turn the steering wheel when the motor vehicle runs at a low speed. Conversely, when the speed of travel of the motor vehicle is increased and there is some steering torque applied, the clutch remains connected and the steering force required is too small when the motor vehicle runs at a high speed. Furthermore, when the steering wheel is turned quickly across its neutral position, the steering torque is instantaneously eliminated. Therefore, the clutch is abrutly connected if the motor vehicle is decelerated, and disconnected if the motor vehicle is accelerated. As a result, the steering stability and the steering feeling they are felt by the driver are impaired.